C# 取消下载StringTaskAsync并重新启动它
我正在使用C# 取消下载StringTaskAsync并重新启动它,c#,asynchronous,task,webclient,cancellation,C#,Asynchronous,Task,Webclient,Cancellation,我正在使用System.Net.WebClient.downloadstringtasksync异步从网页下载字符串 大约5次中有4次成功完成,但偶尔会挂起,我必须重新启动程序以“重置”它 因此,我考虑实施一个超时,在5秒后自动取消任务并重新启动它: async void DoTasks() { string output; int timeout = 5000; WebClient client = new WebClient() { Encoding = Encod
System.Net.WebClient.downloadstringtasksync
异步从网页下载字符串
大约5次中有4次成功完成,但偶尔会挂起,我必须重新启动程序以“重置”它
因此,我考虑实施一个超时,在5秒后自动取消任务并重新启动它:
async void DoTasks()
{
string output;
int timeout = 5000;
WebClient client = new WebClient() { Encoding = Encoding.UTF8 };
Task<string> task = client.DownloadStringTaskAsync(url);
if (await Task.WhenAny(task, Task.Delay(timeout)) == task)
{
output = task.Result;
}
else
{
client.CancelAsync();
DoTasks();
}
}
async void dotask()
{
字符串输出;
int超时=5000;
WebClient=newWebClient(){Encoding=Encoding.UTF8};
Task=client.downloadstringtasksync(url);
if(wait Task.wheny(Task,Task.Delay(timeout))==Task)
{
输出=任务。结果;
}
其他的
{
client.CancelAsync();
DoTasks();
}
}
但是,这会返回一个
NullReferenceException
我不是wait/async方面的专家,我还在学习,但也许你应该这样做
using System;
using System.Net;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Inline code to test");
Console.WriteLine(StartTask(true));
Console.WriteLine("Get actual website to test, write the length of the text only");
Console.WriteLine(StartTask(false).Length);
Console.Write("Press any key");
Console.ReadKey();
}
static string StartTask(bool DoTestInsteadOfGetUrl)
{
var task = DoTasks(DoTestInsteadOfGetUrl);
Task.WaitAny(task);
return task.Result;
}
async static Task<string> DoTasks(bool DoTestInsteadOfGetUrl)
{
int timeout = 20;
int count = 1;
CancellationTokenSource cts;
string output;
do
{
cts = new CancellationTokenSource(timeout);
if (DoTestInsteadOfGetUrl)
{
output = await Task<string>.Run(() => test(count, cts.Token), cts.Token);
}
else
{
WebClient client = new WebClient() { Encoding = Encoding.UTF8 };
output = await Task<string>.Run(() => client.DownloadStringTaskAsync("http://www.google.com"), cts.Token);
}
if (cts.IsCancellationRequested)
{
Console.WriteLine("try #" + count++);
timeout += DoTestInsteadOfGetUrl ? 100 : 50;
}
} while (cts.IsCancellationRequested);
return output;
}
async static Task<string> test(int count, CancellationToken ct)
{
int sleep = 400 - (count * 5);
await Task.Run(() => Task.Delay(sleep), ct);
if (!ct.IsCancellationRequested)
{
Console.WriteLine("succeed #" + count);
}
return sleep.ToString();
}
}
}
使用系统;
Net系统;
使用系统文本;
使用系统线程;
使用System.Threading.Tasks;
命名空间控制台应用程序1
{
班级计划
{
静态void Main(字符串[]参数)
{
WriteLine(“要测试的内联代码”);
Console.WriteLine(StartTask(true));
WriteLine(“让实际的网站进行测试,只写文本的长度”);
Console.WriteLine(StartTask(false).Length);
控制台。写入(“按任意键”);
Console.ReadKey();
}
静态字符串StartTask(bool DoTestInsteadOfGetUrl)
{
var task=dotask(DoTestInsteadOfGetUrl);
Task.WaitAny(Task);
返回任务。结果;
}
异步静态任务点任务(bool DoTestInsteadOfGetUrl)
{
int超时=20;
整数计数=1;
取消源cts;
字符串输出;
做
{
cts=新的CancellationTokenSource(超时);
if(DoTestInsteadOfGetUrl)
{
输出=等待任务.Run(()=>test(count,cts.Token),cts.Token);
}
其他的
{
WebClient=newWebClient(){Encoding=Encoding.UTF8};
输出=等待任务。运行(()=>客户端。下载StringTaskAsync(“http://www.google.com",cts.Token),;
}
如果(cts.IsCancellationRequested)
{
Console.WriteLine(“try#”+count++);
timeout+=DoTestInsteadOfGetUrl?100:50;
}
}while(请求取消cts.iscancellation);
返回输出;
}
异步静态任务测试(整数计数、取消令牌ct)
{
int sleep=400-(计数*5);
等待任务。运行(()=>任务。延迟(睡眠),ct);
如果(!ct.iscancellationrequest)
{
Console.WriteLine(“success#”+count);
}
返回sleep.ToString();
}
}
}
此代码不可能导致NullReferenceException
。它可能在GetTasks
方法中。另外,您没有处理WebClient
。您需要在问题中添加GetTasks
方法。很抱歉,gettask应该是dotask-重新启动该方法没有任何明显的可能导致NullReferenceException
。哪一行引发异常?您还应该发布导致挂起的原始代码。你实际上在问两个不相关的问题。这段代码有很多错误。首先,它不包括OP的WebClient
调用,您需要在dotask
内部进行调用。但是您的dotask
是void
,并且不包含async
修饰符。这目前无法解决OP的问题。@YuvalItzchakov,我更新了代码。。。希望更好